High data rate optical fiber communication is one of the fastest growing areas in the telecommunication industry. Compensation of lightwave polarization effects is one of the key enabling technologies for high-speed and long haul data transmission. There has been much interest in the limitations of high-speed lightwave systems caused by the stochastic nature of polarization mode dispersion (PMD). It has become clear recently that polarization dependent loss (PDL) is also a key limiting factor in high-capacity wavelength multiplexing (WDM) systems, because various optical networking components and erbium doped fiber amplifiers (EDFAs) may produce PDL. Further discussion of these factors is found for example in Lichtman, “Limitations imposed by polarization-dependent gain and loss on all-optical ultralong communication systems,” J. Lightwave Technol., vol. 13, pp. 906–913, 1995; and Huttner, et al., “Polarization-induced distortions in optical fiber networks with polarization-mode dispersion and polarization-dependent losses,” IEEE J. Selected Topics in Quantum Electron., vol. 6, pp. 317–329, 2000.
Some of the deleterious systems issues induced by PDL are: (i) PDL produces a variation in the optical power and signal-to-noise ratio (SNR) of each wavelength channel, (ii) PDL increases the degradation caused by PMD by broadening the distribution of the system power penalty, (iii) given a non-negligible amount of PMD in the transmission fiber, the effects of PDL for many WDM channels will not be correlated with each other, (iv) similar to PMD, the effects of PDL are random and change dynamically in an optical system due to the environment, and (v) PMD compensation becomes much more difficult in the presence of PDL. Therefore, a PDL compensator would be advantageous for high-performance systems.
To date, a functional dynamic PDL compensator has not been demonstrated. Moreover, a practical scheme of fast PDL monitoring that is necessary for dynamic compensation has not been reported. Fast PDL monitoring requires fast polarization scrambling that is independent on the state of polarization (SOP) of incoming optical signals. Existing polarization independent scramblers generate SOP varied non-periodically with time by applying sinusoidal voltages of different frequencies to different waveplates of the scrambler. This scheme limits the PDL measurement time and/or accuracy in the presence of noise.